There have been known audio systems including speakers installed in vehicle seats (for example, see Patent Literatures 1, 2). Such an audio system is able to output sounds in a low-to-high wide frequency range and includes a full-range speaker installed near the headrest of a seat and subwoofers installed in the backrest of the seat or the seat surface. The subwoofers are embedded in the seat and configured to vibrate the seat in accordance with the signal level of a low-frequency band of music. Thus, when a sound outputted from the full-range speaker installed near the headrest and vibration outputted from the subwoofers are combined together, the realism of music can be enhanced.
Subwoofers are not used solely to enhance the realism of music by outputting deep bass (low-frequency sounds). They have been also recently used as means for notifying the driver.
For example, a vehicle notification device has been proposed that when the vehicle departs from the lane, notifies the driver of the departure of the vehicle through vibration. A vehicle notification device has been also proposed that notifies the driver of the position of a lane marking that the vehicle has crossed, through the vibration occurrence position (for example, see Patent Literature 3). As seen above, these vehicle notification devices are able to notify the driver of an alarm about the departure from the lane and whether the vehicle has crossed the left lane marking or crossed the right lane marking, by vibrating the vibrators (subwoofers, etc.) installed in left-right positions of the driver's seat as necessary.
There have been also proposed many other methods of notifying the driver by vibrating speakers or the like installed in the driver's seat.